As publicly known, a spark plug (herein simply referred to as “plug”) that is mounted on a cylinder head of an engine is fixed through a gasket in a manner that its forward-end portion side faces a combustion chamber by screwing the male screw portion on the metal main body side into the female screw portion on the plug hole side. In this case, there is a demand that the orientation (phase position in the rotational direction) of the ground electrode corresponding to the center electrode while facing the combustion chamber, should be adjusted to a particular orientation, from the viewpoints of cancelling the combustion variation among cylinders, improving the ignition quality, improving the fuel consumption, etc.
As a measure for that, there is proposed, for example, a method for producing spark plugs by using a male screw jig for producing spark plugs, as described in Patent Publication 1. According to the technology described in this Patent Publication 1, it is described that a screw-cutting phase angle range α is specified between the arrangement selection position of the ground electrode, which is capable of achieving the optimum ignition performance of the engine, and the origin position of the male screw jig by screwing the male screw jig into the plug hole on the side of the cylinder head. Furthermore, it is described that the joint position or joint-expected position of the ground electrode is defined as a joint phase angle range α0 at a position corresponding to the screw-cutting phase angle range α, which is already specified by using the male screw jig, on the basis of the origin position on the side of the female screw jig, by screwing the female screw jig on the male screw portion on the side of the spark plug.
As publicly known, a plug is equipped beforehand with a gasket. When a plug is tightened and fixed to a plug hole on the side of the cylinder head, the gasket is crushed and deformed to secure a predetermined sealing performance. The actual orientation of the ground electrode is also largely dependent on the quality variation of the gasket and the crushing deformation characteristics. For example, as the crushing deformation characteristics of the gasket change, the degree of tightening of the plug also changes accordingly. Therefore, it is insufficient to conduct only the phase management of the joint position of the ground electrode as described in the above Patent Publication 1, in order to achieve the expected object. Thus, there is yet room for improvement.
On the other hand, it is also possible on the side of plug manufacturers to do a quality assurance to include the crushing deformation characteristics of gaskets. In that case, a gasket-equipped plug is tightened with a specified tightening torque as an inspection before shipping. Therefore, the gasket is every time necessarily subject to the crushing deformation. With this, its reuse becomes impossible. Furthermore, it leads to a result contrary to the fundamental principle of the manufacturers to ship plugs including gaskets under a brand-new condition from the viewpoint of sealing property assurance. Thus, it is not realistic.